Transflective liquid crystal display device

ABSTRACT

An IPS-mode transflective LCD device includes an array of pixels each including a reflective region and a transmissive region juxtaposed. The reflective region operates in a normally-white mode, and the transmissive region operates in a normally-black mode. A common data signal is supplied to the reflective region and transmissive region, whereas the common electrode signal in the transmissive region is an inverted signal of the common electrode signal in the reflective region, to thereby obtain similar gray-scale levels.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a transflective liquid crystal display(LCD) device including a transmissive area and a reflective area in eachpixel of the LCD device.

(b) Description of the Related Art

LCD devices are generally categorized in two types: a transmissive LCDdevice having therein a backlight unit as a light source; and areflective LCD device having therein a reflection film which reflectsexternal light incident onto the LCD device and thus functions as alight source. The reflective LCD device has the advantages of lowerpower dissipation, smaller thickness and lighter weight compared to thetransmissive LCD device, due to absence of a backlight source in thereflective LCD device. On the other hand, the transmissive LCD device issuperior to the reflective LCD device in that the transmissive LCDdevice can be well observed in a dark environment.

There is another type of the LCD device, known as a transflective LCDdevice, which has the advantages of both the reflective and transmissiveLCD devices. Such a transflective LCD device is described in PatentPublication JP-A-2003-344837A, for example. The transflective LCD deviceincludes a transmissive region (or transparent region), and a reflectionregion in each pixel of the LCD device. The transmissive region passeslight emitted from a backlight source, and uses the backlight source asa light source. The reflective region includes a rear reflective plateor reflection film, and uses external light reflected by the reflectionfilm as a light source.

In the transflective LCD device, the image display is performed by thereflective region in a well-lighted environment, with the backlightsource being turned OFF, thereby achieving a smaller power dissipation.On the other hand, the image display is performed by the transmissiveregion in a dark environment, with the backlight source being turned ON,thereby achieving an effective image display in the dark environment.

In general, a variety of modes are used for operating LCD devices,including an in-plane-switching (IPS) mode, a twisted-nematic (TN) mode,and a fringe-field-switching (FFS) mode. Each pixel of the IPS-mode orFFS-mode LCD device includes a pixel electrode and a common electrodewhich are disposed on a common substrate to apply the liquid crystal(LC) layer with a lateral electric field. The IPS-mode or FFS-mode LCDdevice using a lateral electric field rotates the LC molecules in aplane parallel to the substrate to perform the image display, andachieves a higher viewing angle compared to the TN-mode LCD device.

If the IPS mode or FFS mode using a lateral electric field is to beemployed in the transflective LCD device as described above, therearises an image-inversion problem in the LCD device, as described in thepatent publication as mentioned above. More specifically, in a normaldriving technique of the LCD device, if the transmissive region operatesin a normally-black mode wherein absence of the applied voltagecorresponds to a dark state, the reflective region operates in anormally-white mode wherein absence of the applied voltage correspondsto a bright state. The reason of the image-inversion problem will bedescribed in detail hereinafter.

FIG. 34A schematically shows a pixel of a transflective LCD device,which includes therein a reflective region 55 and a transmissive region56. The transmissive region 56 is configured by a first polarizing film51, a first substrate (counter substrate) 61, a LC layer 53 having aretardation of λ/2, a second substrate (TFT substrate) 62, and a secondpolarizing film 52, which are arranged in this order as viewed from thefront of the LCD device 50, wherein λ is a wavelength of the light. Thereflective region 55 is configured by the first polarizing film 51,first substrate 61, LC layer 53 having a retardation of λ/4, aninsulation film 63, and a reflection film 54, as effective constituentelements. In FIG. 26A, polarizing axis of the polarizing films 51, 52,longer axis of the LC molecules in the LC layer 53 are depicted in thestate wherein the LCD device is rotated by 90 degrees along a planenormal to the sheet of the drawing in the counterclockwise direction asviewed from the left of the drawing.

FIG. 34B shows polarization of light in the respective regions 55, 56 inFIG. 34A for the case of presence (Von) and absence (Voff) of theapplied voltage, in the portions wherein the light passes through thefirst polarizing film 51, LC layer 53 and second polarizing film 52. InFIG. 34B, an arrow means linearly-polarized light, “L” encircled by acircle means counterclockwise-circularly-polarized light, “R” encircledby a circle means clockwise-circularly-polarized light, blank elongatebar means the director of the LC, i.e., longer axis of the LC molecules.FIG. 35 shows a sectional view of this type of the practical LCD device,the principle of which is shown in FIGS. 26A and 26B, including abacklight source 57.

In the LCD device 50 a shown in FIG. 35, the reflective region 55 usesthe reflection film 54 as the light source, whereas the transmissiveregion 56 uses the backlight source 57 as the light source.

The first polarizing film 51 disposed at the front side of the LC layer53 and the second polarizing film 52 disposed at the rear side thereofhave respective polarizing axes, which are perpendicular to one another.The LC layer 53 includes LC molecules having a director which is 90degrees deviated from the polarizing axis of the second polarizing film52 upon absence of the applied voltage. Assuming that the polarizingaxis of the second polarizing film 52 is directed at a referencedirection (zero degree), for example, the polarizing axis of the firstpolarizing film 51 is directed at 90 degrees and the longer axis of theLC molecules in the LC layer 53 is also directed at 90 degrees. Thezero-degree direction is shown as the lateral direction in FIG. 34B, andthe 90-degree direction is shown as the vertical direction in FIG. 34B.The cell gap of the LC layer 53 in the transmissive region 56 isadjusted such that the retardation Δnd is equal to λ/2, whereas the cellgap of the LC layer 53 in the reflective region 55 is adjusted such thatthe retardation Δnd is equal to λ/4, given λ, Δn and d being wavelengthof the light, refractive-index anisotropy and cell gap, respectively. Asfor λ, if the wavelength of green light is used as a reference, λ is 550nm.

Operation of the LCD device shown in FIGS. 34A, 34B and 35 will bedescribed hereinafter, for each case of absence and presence of theapplied voltage in respective regions 55, 56.

(1) Reflective Region Upon Absence of Applied Voltage:

In the left column (Voff) of the reflective region 55 shown in FIG. 34B,a linearly-polarized light polarized at 90 degrees, i.e., 90-degreelinearly-polarized light is incident onto the LC layer 53 after passingthrough the first polarizing film 51. Since the optical axis of thelinearly-polarized light incident onto the LC layer 53 is aligned withthe longer axis of the LC molecules, the 90-degree linearly-polarizedlight passes through the LC layer 53 as it is, and is then reflected bythe reflection film 54. The linearly-polarized light does not change thestate thereof in general after the reflection, as shown in FIG. 34B, andis again incident onto the LC layer 53 as the 90-degreelinearly-polarized light. The 90-degree linearly-polarized light passesthrough the LC layer 53 as it is and is incident onto the firstpolarizing film 51, which has a polarizing axis at 90 degrees, passesthe 90-degree linearly-polarized light as it is. Thus, absence of theapplied voltage allows the reflective region to assume a bright state.

(2) Reflective Region Upon Presence of Applied Voltage:

In the right column (Von) of the reflective region 56 in FIG. 34B, the90-degree linearly-polarized light passed by the first polarizing film51 is incident onto the LC layer 53. The voltage applied to the LC layer53 directs the longer axis of the LC molecules from zero degree to 45degrees within the plane parallel to the substrates. The deviation ofpolarized direction of the incident linearly-polarized light from thelonger axis of the LC molecules in the LC layer 53 by 45 degrees and theretardation of λ/4 change the 90-degree linearly-polarized light into aclockwise-circularly-polarized light after the reflection, which isincident onto the reflection film 54 and reflected thereby. Thereflected light shifts to a counterclockwise-circularly-polarized lightand is incident onto the LC layer 53. Thecounterclockwise-linearly-polarized light is changed by the LC layer 53into a zero-degree linearly-polarized light and incident onto the firstpolarizing film 51. The polarizing film 51 having a polarizing axis at90 degrees blocks the incident light, thereby representing dark state.

Thus, the reflective region 55 operates in a normally-white mode whereinabsence of the applied voltage provides a bright state, whereas presenceof the applied voltage provides a dark state.

(3) Transmissive Region Upon Absence of Applied Voltage:

In the left column of the transmissive region 56 shown in FIG. 34B, azero-degree linearly-polarized light is passed by the second polarizingfilm 52 and incident onto the LC layer 53. Since this incident light hasa polarized direction normal to the longer axis of the LC molecules inthe LC layer 53, the incident light is passed by the LC layer 53 as itis, and is incident onto the first polarizing film 51 as the zero-degreelinearly-polarized light. The first polarizing film 51 having apolarizing axis at 90 degrees blocks the incident light, therebyrepresenting a dark state.

(4) Transmissive Region Upon Presence of Applied Voltage:

In the right column of the transmissive region 56 shown in FIG. 34B, azero-degree linearly-polarized light is passed by the second polarizingfilm 52 and incident onto the LC layer 53. The voltage applied to the LClayer 53 directs the longer axis of the LC molecules from zero degree to45 degrees within the plane parallel to the substrates. The deviation ofpolarized direction of the incident linearly-polarized light from thelonger axis of the LC molecules in the LC layer 53 by 45 degrees and theretardation of λ/2 of the LC layer change the zero-degreelinearly-polarized light into a 90-degree linearly-polarized light,which is incident onto the first polarizing film 51. The firstpolarizing film 51 having a polarizing axis at 90 degrees passes theincident light, thereby representing a bright state.

Thus, the transmissive region operates in a normally-black mode whereinabsence of the applied voltage provides a dark state whereas presence ofthe applied voltage provides a bright state.

The image-inversion problem is a general problem common to thelateral-electric-field modes (IPS mode, FFS mode) and other LCD modes.However, as to the TN mode, horizontal-orientation mode (ECB mode) orvertical-alignment mode (VA mode), for example, the image-inversionproblem may be solved using a circularly-polarized light as the incidentlight to the LC layer. For this purpose, the orientations of the firstpolarizing film and λ/4 wavelength film are deviated by 45 degrees fromone another. However, if the incident light is a circularly-polarizedlight, the circularly-polarized light looses the sensitivity to therotation of the LC molecules parallel to the substrates, and thus passesthrough the LC layer as the circularly-polarized light. Accordingly, theLCD device using the lateral electric field represents a dark state atany time irrespective of presence or absence of the applied voltage ineither of the reflective mode and the transmissive mode. That is, thelateral-electric-field-mode LCD device cannot represent the imagethereof by using such a λ/4 wavelength film.

As described above, the transflective LCD device has the problem thatboth the absence and presence of the applied voltage provide reversedimages of bright state and dark state in each pixel. The patentpublication as mentioned above solves this problem without using the λ/4wavelength film, by using the arrangement shown in FIG. 35, wherein thepolarizing axis of the first polarizing film 51 is 45 degrees deviatedfrom the longer axis of the LC molecules in the LC layer 53, as shown onthe left side of the drawing. In this case, the reflective region 55operates in a normally-black mode, whereas the transmissive region 56operates in a normally-white mode. In order for changing thetransmissive region 56 to operate in a normally-black mode, a λ/2wavelength film 58 is interposed between the second polarizing film 52and the LC layer 53, the λ/2 wavelength film 58 having an optical axisat 135 degrees, which is perpendicular to the longer axis of the LCmolecules in the LC layer 53.

By using the above configuration, in the front viewing angle, the λ/2wavelength film 58 compensates the polarizing effect on the light by theLC layer 53 having a retardation at λ/2. Thus, the combination of the LClayer 53 and λ/2 wavelength film 58 provides a substantially similarpolarized state for both the incident light and the reflected light.Accordingly, the light passed by the second polarizing film 52 andassuming a 90-degree linearly-polarized state remains in the samepolarized state after passing through the λ/2 wavelength film 58 and LClayer 53, and thus cannot pass through the first polarizing film 51. Inshort, the λ/2 wavelength film 58 interposed between the LC layer 53 andthe second polarizing film 56 allows the transmissive region 56 tooperate in a normally-white mode.

In the LCD device 50 a shown in FIG. 35, the polarized direction of thelight incident onto the LC layer 53 is deviated from the parallel ornormal direction of the longer axis of the LC molecules in the LC layer53. This involves a significant leakage of light during display of adark state, due to the wavelength dispersion characteristic of theretardation of the LC layer 53. In addition, the λ/2 wavelength film 58itself has a wavelength dispersion characteristic, which also causesleakage light during display of a dark state.

It is to be noted that the image-inversion problem, wherein thetransmissive region 56 and the reflective region operate in reversenormal modes, can be solved by inverting the polarity of the appliedvoltage between the transmissive region 56 and the reflective region 55.The inversion of the voltage polarity as used herein is such thatabsence of the applied voltage in the transmissive region 56 andpresence of the applied voltage in the reflective region 55 areconcurrently performed. However, this configuration is not known in thefield of LCD devices. In addition, the problem encountered in such aconfiguration and the technique for solving the problem are also notknown.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a transflective LCDdevice which is capable of solving the image-inversion problemencountered in the conventional transflective LCD device due to, forexample, the normally-white mode of the transmissive region and thenormally-black mode of the reflective region, by providing differentvoltages to the LC layer in the reflective region and the transmissiveregion.

It is another object of the present invention to provide a method fordriving a transflective LCD device having a reflective region and atransmissive region in each of the pixels.

The present invention provides in a first aspect thereof a liquidcrystal display (LCD) device including first and second polarizing filmshaving polarizing axes perpendicular to one another, a liquid crystal(LC) layer interposed between the first polarizing film and the secondpolarizing film, the LC layer defining an array of pixels each includinga reflective region and a transmissive region juxtaposed, the pixelsbeing driven by a lateral electric field, wherein:

LC molecules of the LC layer have a longer axis extending parallel to ornormal to light incident onto the LC layer in the reflective region; and

each of the pixels includes a pixel electrode receiving a pixel signalwhich is common between the reflective region and the transmissiveregion, a first common electrode receiving a first common signal whichis common among the reflective regions of a plurality of the pixels, anda second common electrode receiving a second common signal which iscommon among the transmissive regions of the plurality of the pixels.

The present invention provides, in a second aspect thereof, atransflective liquid crystal display (LCD) device including: a liquidcrystal (LC) layer defining an array of pixels arranged in a matrix,each of the pixels including therein a reflective region and atransmissive region juxtaposed, wherein:

each of the pixels includes a first pixel electrode in the reflectiveregion, and a second pixel electrode in the transmissive region; and

each of the pixels is associated with a first switching device forcoupling together the first electrode and a data line supplying a datasignal, and a second switching device for coupling together the secondelectrode and the data line.

The present invention provides, in a third aspect thereof, a method fordriving a transflective liquid crystal display device (LCD) including areflective region and a transmissive region in each of pixels arrangedin an array, said method comprising the steps of:

generating a first data signal and a second data signal havingtherebetween a specific potential relationship; and

applying said first data signal and said second data signal to saidreflective region and said transmissive region, respectively.

The above and other objects, features and advantages of the presentinvention will be more apparent from the following description,referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a pixel in a transflective LCDdevice according to a first embodiment of the present invention.

FIG. 2 is a schematic top plan view of the pixel shown in FIG. 1.

FIG. 3A is a waveform diagram of a driving signal applied in thereflective region of the pixel of FIG. 1, and FIG. 3B is a waveformdiagram of a driving signal applied in the transmissive region of thepixel of FIG. 1, both in a specific frames.

FIGS. 4A and 4B schematically show polarized state of the light inportions of the reflective region and transmissive region, respectively,applied with driving signals shown in FIGS. 3A and 3B.

FIGS. 5A and 5B are waveform diagrams showing, similarly to FIGS. 3A and3B, respectively, driving signals in frames different from the specificframes shown in FIGS. 3A and 3B.

FIGS. 6A and 6B schematically show, similarly to FIGS. 4A and 4B,polarized state of the light in portions of the reflective region andtransmissive region.

FIG. 7A shows potential change of the pixel electrode and commonelectrode disposed in the reflective region, and FIG. 7B shows potentialchange of the pixel electrode and common electrode disposed in thetransmissive region.

FIGS. 8A and 8B each show a potential distribution together with aleakage-light distribution by using isoelectric line andiso-transmittance line, which are obtained by a simulation.

FIG. 9 is a sectional view of the reflective film in the vicinity of thepixel electrode or common electrode.

FIG. 10A is a top plan view of a TFT substrate in a step of fabricationprocess thereof, and FIGS. 10B to 10D are sectional views taken alonglines A-A′, B-B′ and C-C, respectively, in FIG. 10A.

FIG. 11A is a top plan view of the TFT substrate in a step subsequent tothe step shown in FIG. 10A, and FIG. 11B is a sectional view taken alongline D-D′ in FIG. 11A.

FIG. 12A is a top plan view of the TFT substrate in a step subsequent tothe step shown in FIG. 11A, and FIGS. 12B to 12D are sectional viewstaken along lines corresponding to lines A-A′, B-B′ and C-C′,respectively, in FIG. 10A.

FIG. 13A is a top plan view of the TFT substrate in a step subsequent tothe step shown in FIG. 12A, and FIGS. 13B to 13D are sectional viewstaken along lines corresponding to lines A-A′, B-B′ and C-C′,respectively, in FIG. 10A.

FIG. 14A is a top plan view of the TFT substrate in a step subsequent tothe step shown in FIG. 13A, and FIGS. 14B to 14D are sectional viewstaken along lines corresponding to lines A-A′, B-B′ and C-C′,respectively, in FIG. 10A.

FIG. 15A is a top plan view of the TFT substrate in a step subsequent tothe step shown in FIG. 14A, and FIGS. 15B to 15D are sectional viewstaken along lines corresponding to lines A-A′, B-B′ and C-C′,respectively, in FIG. 10A.

FIG. 16A is a top plan view of the TFT substrate in a step subsequent tothe step shown in FIG. 15A, and FIG. 16B is a sectional view taken alongline E-E′ in FIG. 16A.

FIG. 17A is a top plan view of the TFT substrate in a step subsequent tothe step shown in FIG. 16A, and FIGS. 17B to 17D are sectional viewstaken along lines corresponding to lines A-A′, B-B′ and C-C′,respectively, in FIG. 10A.

FIG. 18 is a schematic top plan view of a LCD device according to asecond embodiment of the present invention.

FIG. 19 is a schematic block diagram of the LCD device shown in FIG. 18.

FIG. 20 is a driving-signal waveform diagram in the LCD device shown inFIG. 18.

FIG. 21 is a schematic top plan view of a LCD device according to athird embodiment of the present invention.

FIG. 22 is a schematic block diagram of the LCD device shown in FIG. 21.

FIG. 23 is a driving-signal waveform diagram in the LCD device shown inFIG. 21.

FIG. 24 is a schematic sectional view of a transflective LCD deviceaccording to a fourth embodiment of the present invention.

FIG. 25 is a table tabulating the angle combination for the opticaltransmission axis of the polarizing films, longer axis of the LCmolecules, and optical axis of the λ/2 wavelength films.

FIG. 26 is a graph showing the relationship obtained by simulationbetween the optical transmission and wavelength of the light in thetransmissive region.

FIG. 27 is a schematic diagram showing the image represented in thetransflective LCD device of the first embodiment.

FIGS. 28A and 28B are diagrams showing the viewing angle dependency ofthe luminance and contrast ratio by using iso-luminance line andiso-contrast line, obtained by simulation in the case of using asingle-axial wavelength film.

FIGS. 29A and 29B are diagrams showing the viewing angle dependency ofthe luminance and contrast ratio by using iso-luminance line andiso-contrast line, obtained by simulation in the case of using acombination wavelength film.

FIGS. 30A and 30B are diagrams showing the viewing angle dependency ofthe luminance and contrast ratio by using iso-luminance line andiso-contrast line, obtained by simulation in the case of using a biaxialwavelength film.

FIG. 31 is a sectional view of a reflective film in the vicinity of thepixel electrode (or common electrode) in the reflective region.

FIG. 32 is a sectional view of a FFS-mode LCD device to which thepresent invention can be applied in the above embodiments.

FIG. 33 is a sectional view of the IPS-mode LCD device of the firstembodiment.

FIG. 34A is a sectional view of a conventional transflective LCD device,and FIG. 34B is a schematic diagram of the LCD device of FIG. 34A.

FIG. 35 is a sectional view of another conventional transflective LCDdevice described in a patent publication

PREFERRED EMBODIMENT OF THE INVENTION

Now, the present invention is more specifically described with referenceto accompanying drawings, wherein similar constituent elements aredesignated by similar reference numerals.

FIG. 1 is a sectional view schematically showing a pixel in atransflective LCD device according to a first embodiment of the presentinvention. FIG. 2 is a schematic top plan view of the TFT substrate inthe pixel shown in FIG. 1. The LCD device, generally designated bynumeral 10, includes a first polarizing film 11, counter substrate(first substrate) 12, a LC layer 13, a TFT substrate (second substrate)14, and a second polarizing film 15, which are arranged in this orderfrom the front side toward the rear side of the LCD device 10. The firstpolarizing film 11 has a optical transmission axis at 90 degrees, andthus an absorption axis at zero degree, whereas the second polarizingfilm 15 has an optical transmission axis at zero degree, and thus anabsorption axis of 90 degrees. The LC layer 13 includes LC moleculeshaving a longer axis at 90 degrees upon absence of the applied voltage,in this example.

Each pixel of the LCD device 10 includes a reflective region 21 and atransmissive region 22. The reflective region 21 includes therein areflection film 16 and a transparent insulation film 17, which areconsecutively formed on the TFT substrate 14. The reflection film 16reflects light passed by the first polarizing film 11 toward the same.The reflection film 16 has a concave/convex (uneven) surface forachieving a higher dispersion of the reflected light. On the insulationfilm 17, there are provided a first pixel electrode 35 and a firstcommon electrode 37 for driving the LC layer 13 in the lateraldirection. On the transmissive region 22, there are also provided asecond pixel electrode 36 and a second common electrode 38 on the TFTsubstrate 14 for driving the LC layer 13 in the lateral direction.

The reflective region 21 uses the light reflected by the reflection film16 as a light source. The LCD device 10 includes a backlight source (notshown) at the rear side of the second polarizing film 15, which is usedin the transmissive region 22 as a light source. In the transmissiveregion 22, the cell gap is adjusted such that the LC layer 13 has aretardation substantially equal to λ/2. The term “substantially” as usedherein means that an actual retardation equal to (α+(λ/2)) provides aneffective retardation of λ/2. This is because the rotation of the LCmolecules is suppressed in the vicinity of the substrates 12, 14 uponapplication of a voltage, although the LC molecules in the central areaof the cell gap rotates corresponding to the applied voltage. Forexample, if the LC layer 13 has a retardation of Δnd=300 nm, theeffective retardation Δndeff upon application of a voltage isΔndeff=λ/2=550 nm/2=275 nm. On the other hand, in the reflective region21, the cell gap is adjusted such that the effective retardation of theLC layer 13 upon application of a voltage assumes λ/4, by selecting anoptimum thickness for the insulation film 17.

As shown in FIG. 2, the TFT substrate 14 mounts thereon a plurality ofgate lines 31 extending in a row direction and a plurality of data lines32 extending in a column direction of the TFT substrate 14. TFTs 33 and34 are disposed corresponding to the reflective region 21 and thetransmissive region 22, respectively, in the vicinity of each of theintersections between the gate lines 31 and the data lines 32. The TFTs33, 34 each have a gate electrode connected to a common gate line 31, asource and a drain, one of which is connected to a common data line 32,and the other of which is connected to a corresponding pixel electrode35 or 36.

The first and second common electrodes 37 and 38 correspond to thereflective region 21 and the transmissive region 22, respectively. Eachcommon electrode 37, 38 in the pixel includes a bus line extendingparallel to the gate line 31, and a plurality of branch lines extendingtoward the internal of the pixel area from the bus line. The firstcommon electrode 37 opposes the first pixel electrode 35 in thereflective region 21, whereas the second common electrode 38 opposes thesecond pixel electrode 36 in the transmissive region 22. The first andsecond common electrodes 37, 38 are applied with respective drivingsignals corresponding to the reflective region 21 and transmissiveregion 22.

The first and second pixel electrodes 35, 36 are connected to respectiveTFTs 33, 34, which are connected to a common gate line 31 and a commondata line 32 for receiving a common gate signal and a common data signal(pixel signal). Thus, both the pixel electrodes 35, 36 receive a commondata signal at the same timing. In the reflective region 21, theorientation in the LC layer 13 is controlled by the lateral electricfield caused by the potential difference between the pixel electrode 35and the common electrode 37, whereas in the transmissive region 22, theorientation in the LC layer 13 is controlled by the lateral electricfield caused by the potential difference between the pixel electrode 36and the common electrode 38. The reason for providing separate pixelelectrodes 35 and 36 and separate TFTs 33 and 34 in respective regions21, 22 of the pixel, irrespective of writing the same data signal intothe pixel electrodes 35 and 36, is that the transient potential isdifferent between the pixel electrodes 35 and 36 after turn-off of theTFTs 33, 34, which will be detailed later.

FIG. 3A shows a driving waveform diagram showing the signal potential ofthe pixel electrode 35 and common electrode 37 in the reflective region21 at a specific stage of operation, and FIG. 3B shows the signalpotential for the pixel electrode 36 and common electrode 38 in thetransmissive region 22 at the same stage. As shown in these figures, thesignal potential of the first and second common electrodes 37 38 isinverted at a specific timing between zero volt and 5 volt, for example,and the signal potential of the first common electrode 37 is invertedfrom the signal potential of the second common electrode 38.

The pixel electrodes 35, 36 are applied with any desired signalpotential between zero volt and 5 volts, for example. The pixelelectrodes 35, 36, which are connected to the common data line 32,receive a common data signal. As exemplified in FIG. 3A, when the pixelelectrode 35 is applied with a zero-volt data and the common electrode37 is applied with a 5-volt data in an i-th frame, the potentialdifference between the pixel electrode 35 and the common electrode 37assumes 5 volts. Thus, the LC layer 13 in the reflective region 21 isdriven by 5 volts. In the same i-th frame, as shown in FIG. 3B, thepixel electrode 36 is applied with the zero-volt data signal and thecommon electrode 38 is applied with a zero-volt data, whereby thepotential difference therebetween assumes zero volt. Thus, the LC layer13 in the transmissive region 22 is not driven, i.e., driven by zerovolt.

FIGS. 4A and 4B show polarized state of the light in the respectiveportions in the reflective region 21 and transmissive region 22,respectively, when the LC device 10 is applied with the respectivedriving signals shown in FIGS. 3A and 3B. Upon application of thedriving signal shown in FIG. 3A, the orientation of the LC layer 13 inthe reflective region 21 is rotated by 45 degrees due to the potentialdifference between the pixel electrode 35 and the common electrode 37.Thus, as shown in the left column of FIG. 4A, the 90-degreelinearly-polarized light passed by the first polarizing film 11 changesthe polarized state thereof after passing through the LC layer 13 tothereby shift to a counterclockwise-circularly-polarized light. Thecounterclockwise-circularly-polarized light is reflected by thereflection film 16 to shift to a clockwise-circularly-polarized light,as shown by the right column of FIG. 4A, again passed by the LC layer 13to shift to a zero-degree linearly-polarized light, and incident ontothe first polarizing film 11. The first polarizing film 11 blocks thezero-degree linearly-polarized light, thereby representing a dark statein the reflective region 21.

On the other hand, as shown in FIG. 4B, absence of the electric fielddue to a zero potential difference between the pixel electrode 36 andthe common electrode 28 allows the orientation of the LC layer 13 in thetransmissive region 22 to remain at 90 degrees. Thus, the zero-degreelinearly-polarized light passed by the second polarizing film 15maintains the polarized state thereof after passing through the LC layer13, and is incident onto the first polarizing film 11, which blocks theincident light, thereby representing a dark state in the transmissiveregion 22.

As described above, by applying an inverted signal and a non-invertedsignal to the first and second common electrodes 37, 38, a common datasignal applied to both the pixel electrodes 35, 36 is sufficient forrepresenting a dark state in both the reflective region 21 andtransmissive region 22. This is because the inverted signal andnon-inverted signal allow the orientation of the LC layer 53 to berotated by 45 degrees only in the reflective region 21. Thus, both thereflective region 21 and transmissive region 22 assume a dark statewithout the necessity of applying different data signals.

FIGS. 5A and 5B each show, similarly to FIGS. 3A and 3B, a drivingwaveform signal at another stage of operation. FIGS. 6A and 6B show,similarly to FIGS. 4A and 4B, polarized state of light at the anotherstage. In the another stage shown in FIG. 5A, the signal potentialapplied in the j-th frame between the pixel electrode 35 and the commonelectrode 36 provides no electric field to the LC layer 13 in thereflective region 21, whereby the orientation of the LC layer 13 in thereflective region 21 remains at 90 degrees. Thus, as shown in FIG. 6A,the 90-degree linearly-polarized light passed by the first polarizingfilm 11 passes through the LC layer 13 in the reflective region 21 as itis, is reflected by the reflection film 16, passes through the LC layer13, and is incident onto the first polarizing film 11 without changingthe polarized state thereof. Thus, the polarizing film 11 passes thelight to represent a bright state in the reflective region 21.

On the other hand, in the j-th frame shown in FIG. 5B, the orientationof the LC layer 13 in the transmissive region 22 is rotated by 45degrees due to the electric field formed by the potential differencebetween the pixel electrode 36 and the common electrode 38. Thus, asshown in FIG. 6B, the 90-degree linearly-polarized light passed by thesecond polarizing film 15, passes through the LC layer 13 in thetransmissive region 22 to shift to a 90-degree linearly-polarized light,and is incident onto the first polarizing film 11. The first polarizingfilm 11 passes the incident light to represent a bright state in thetransmissive region 22. Thus, both the reflective region 21 andtransmissive region 22 assume a bright state without the necessity ofapplying different data signals.

FIGS. 7A and 7B show the transient potential of the pixel electrodes 35and 36, respectively, after applying the data signal. In the case of agate-line-inversion driving scheme for the LCD device 10 shown in thesefigures, the polarity of the driving signal is inverted at every frameend for each pixel, and two adjacent rows receive opposite polarities.After a gate signal pulse Vg is applied to the gate line 31 and removedtherefrom, the potential polarity of the common electrodes 37, 38repeats inversion at every frame by responding to the polarity inversionof the driving signal in each row until a next gate signal pulse isapplied to the gate line 31.

Since the TFTs 33, 34 are turned OFF during this interval, the pixelelectrodes 35, 36 are isolated from the data line 32 and reside in afloating state. Thus, the potential of the pixel electrodes 35, 36fluctuates as shown in the figures due to the capacitive couplingbetween the pixel electrodes 35, 36 and the common electrodes 37, 38,while maintaining the initial potential differences P1, P2 at the timeof writing the data signal into the pixel electrodes 35, 36. In thiscase, the situation of the potential fluctuation is different betweenthe pixel electrode 35 and the pixel electrode 36 after the writing ofdata signal into the pixel electrodes 35, 36, as will be understood fromFIGS. 7A and 7B.

In the present embodiment, the common electrode of a pixel is separatedinto the first and second common electrodes 37 and 38 corresponding tothe reflective region 21 and the transmissive region 22, respectively.The inverted and non-inverted signals applied to these common electrodes37, 38 allow the electric fields applied to the LC layer 13 in thereflective region 21 and the transmissive region 22 to have oppositemagnitudes so that the same gray-scale-level is obtained both in thereflective region 21 and the transmissive region 22. The term “oppositemagnitudes” as used herein means that when one of the regions has alarger (maximum, for example) electric field, the other of the regionshas a corresponding lower (minimum, for example) electric filed. Thus,the reflective region 21 and the transmissive region 22 of each pixelare applied with the same data signal to represent the same gray-scalelevel in the image, whereby the image-inversion problem encountered inthe conventional IPS-mode LCD device can be solved without employing acomplicated signal scheme.

In the present embodiment, the orientation of the LC layer 13 in thetransmissive region 21 during display of a dark state is parallel ornormal to the polarized direction of the light incident onto the LClayer 13. This reduces the adverse influence by the wavelengthdispersion characteristic of the LC layer 13 on the image during displayof a dark state, whereby leakage light is reduced during the display ofa dark state. The relationship between the first and second polarizingfilms 11, 15 and the orientation of the LC layer 23 in the transmissiveregion 22 is similar to that in the typical transmissive IPS-mode LCDdevice, whereby a contrast ratio in the transmissive region 22 in thepresent embodiment is similar to that achieved in the typicaltransmissive IPS-mode LCD device.

In the typical TN-mode LCD device, the reflection film is generallyconfigured as a reflective pixel electrode, which is applied with a datasignal for driving the LC layer corresponding to a desired gray-scalelevel. On the other hand, in the IPS-mode LCD device, the LC layer isdriven by the electric field applied by the pixel electrode and thecommon electrode. This allows the reflection film 16 to be applied withany desired voltage. The influence by the potential of the reflectionfilm 16 on the image will be discussed hereinafter.

FIGS. 8A and 8B show an electric field distribution and an opticaltransmittance distribution obtained by a simulation in the reflectiveregion 21 in the case of the reflection film 16 being applied with 2.5volts and 5 volts, respectively, with the pixel electrode 35 and commonelectrode 37 being fixed at 5 volts and zero volt, respectively.

If the potential of the reflection film 16 is a median between thepotential of the pixel electrode 35 and the potential of the commonelectrode 37, as shown in FIG. 8A, a significant leakage light isobserved in the area of the pixel electrode 35 and the common electrode37 due to a higher transmittance of the LC layer in this area; however,a lower leakage light is observed in the gap between the pixel electrode35 and the common electrode 37. On the other hand, if the reflectionfilm 16 is equi-potential with the common electrode 37, as shown in FIG.8B, a significant leakage light is observed in the area of the commonelectrode 37 due to a higher transmittance in this area. The reason forthe optical transmittance distribution in the latter case is possiblythat a higher electric field between the pixel electrode 35 and thereflection film 16 directs the electric field (electric flux line),which would otherwise converge to the common electrode 37, toward thereflection film 16, and thus the electric field for driving the LCmolecules in the area of the common electrode 37 is insufficient.

As understood from the above results of simulation, the potential of thereflection film 16 is a median between the pixel electrode 35 and thecommon electrode 37. The potential of the reflection film 16 may bedirectly controlled by applying a specific voltage, or may be indirectlycontrolled by a capacitive coupling while floating the potential of thereflection film 16. If the capacitive coupling is to be employed, forexample, a first interconnect applied with the equi-potential with thepixel electrode 35 and a second interconnect applied with theequi-potential with the common electrode 37 are provided on the rearside of the reflection film 16 so that the area ratio of the firstinterconnect to the second interconnect is set at 1:1, whereby thepotential of the reflection film 16 assumes the median.

As shown in FIG. 8A, the median potential of the reflection film 16incurs a significant leakage light in the area of the pixel electrode 35and the common electrode 37, which is undesirable because a higheroptical transmittance occurs therein during display of a dark state. Forsuppressing the adverse influence by the leakage light on the image, apattern configuration wherein the reflection film 15 does not have aportion overlapping the pixel electrode 35 and the common electrode 37as observed normal to the substrate may be employed, as shown in FIG. 9.This configuration reduces the luminance of the reflected light observedin the area of the pixel electrode 35 and the common electrode 37 duringdisplay of a dark state.

A process for manufacturing the TFT substrate in the LCD device of FIG.1 will be described hereinafter with reference to FIGS. 10A to 17Ashowing top plan view in consecutive steps of fabrication and additionalsectional views. The additional sectional figures depict the reflectiveregion 21, transmissive region 22 and boundary between the reflectiveregion 21 and the transmissive region 22, and are designated by anumeral equal to the numeral of the corresponding top plan views andattached with alphabetic symbols following to the alphabetic symbol “A”in the order of the alphabetic symbols shown in the corresponding topplan views. For example, FIGS. 10B, 10C and 10D are sectional viewstaken along lines A-A′ in the reflective region 21, B-B′ in thetransmissive region 22, and C-C′ in the boundary, respectively, shown inFIG. 10A.

First, gate lines 31, first common electrode lines 37 a and secondcommon electrode lines 38 a are formed as shown in FIGS. 10A to 10D. Inthis step, the first common electrode lines 37 a are formed to extendtoward the reflective region 21 from the bus line for providing apotential to the reflection film 15. The gate lines 31, first commonelectrode lines 37 a and second common electrode lines 38 a are thencovered with an insulation film deposited thereon.

Subsequently, as shown in FIG. 11A, a semiconductor layer 39 is formedwhich later configures source/drain regions of the TFTs 33. In thisstep, as shown in FIG. 11B, the semiconductor layer 39 is formed tooverlap the gate lines (or gate electrodes) 31. Thereafter, pixelelectrode lines 35 a connected to the source/drain regions of the TFTs33 and pixel electrode lines 36 a connected to the source/drain regionsof the TFTs 34 are formed, as shown in FIGS. 12A to 12D.

In the reflective region 2, one of the first common electrode lines 37 ais interposed between two adjacent pixel electrode lines 35 a as viewednormal to the substrate. The area ratio of the first common electrodelines 37 a to the pixel electrode lines 35 a is set at 1:1 in the pixel.This allows the reflection film 16 to assume a median potential betweenthe pixel electrode 35 and the first common electrode 37. The pixelelectrodes 35, 36 are then covered by an insulator film depositedthereon.

Subsequently, an overcoat layer 40 having a convex/concave surface isformed on the reflective area 21 and a periphery of the transmissivearea 22, as shown in FIGS. 13A to 13D. An aluminum film is deposited onthe entire surface and patterned to form reflection film 16 in thereflective region 1. The reflection film 16 ha a slit at the center ofeach pixel electrode line 35 a and each first common electrode linesq5C.

After forming the reflection film 16, a flat overcoat film 41 is formedthereon having a pattern shown in FIG. 15A in the substantially entirearea of the pixel. The flat overcoat film 41 has a step portion betweenthe reflective region 21 and the transmissive region 22, as shown inFIGS. 15B to 15D, thereby adjusting the difference of the cell gaptherebetween. Subsequently, contact holes 42 are formed in the insulatorfilm to expose the pixel electrode lines 35 a, 36 a, first commonelectrode lines 37 a, second common electrode line 38 a, as shown inFIGS. 15A and 15B.

After forming the contact holes 42, the pixel electrodes 35, 36, firstcommon electrode 37, second common electrode 38 are formed on the flatovercoat film in a pattern shown in FIG. 17A. The section of thereflective region 21, transmissive region and the boundary therebetweenare shown in FIGS. 17B, 17C and 17D, respectively. The pixel electrode35, 36, first common electrode 37 and second common electrode 38 areconnected to the pixel electrode line 35 a, 36 a fits common electrodeline 37 a, and second common electrode lines 38 a, rex, via respectivecontact holes 42. Thus, the TFT substrate 14 for use in thetransflective LCD device of the present embodiment is obtained.

FIG. 18 is a top plan view of a TFT substrate, showing a pixel of atransflective LCD device according to a second embodiment of the presentinvention. The LCD device of the present embodiment, generallydesignated by numeral 10 a, has a sectional structure similar to that ofthe LCD device 10 of the first embodiment, and includes first polarizingfilm, counter substrate, LC layer, TFT substrate, and second polarizingfilm. The polarizing axis of the first and second polarizing films aswell as the orientation of the LC layer in the present embodiment isalso similar to that in the first embodiment. The LCD device of thepresent embodiment is different from the LCD device of the firstembodiment in the planar structure of the pixel, and the scheme ofsignal transfer via the gate lines 31 and data lines 32.

As understood from FIG. 18, a plurality of gate lines 31 extending inthe row direction and a plurality of data lines 32 extending in thecolumn direction are formed on the TFT substrate. TFTs 33, 34 areprovided in the vicinity of each of the intersections between the gatelines 31 and data lines 32. The gate lines 31 for each row of the pixelsinclude a gate line 31 a connected to the gate of the TFTs 33, and agate line 31 b connected to the gate of the TFTs 34. The TFTs 33 eachhave a source/drain path connected between a data line 32 and the firstpixel electrode 35 provided in the reflective region 21, whereas theTFTs 34 each have a source/drain path connected between the same dataline 32 and the second pixel electrode 36 in the transmissive region 22.The common electrode 39 formed in common to the reflective region 21 andtransmissive region 22 is connected to a single common electrode (COM)line 40, which supplies a common electrode signal to all the pixels ofthe LCD device 10 a.

FIG. 19 shows the overall configuration of the LCD device 10 a of FIG.18 including a LCD driver 101. The LCD device 10 a includes, forexample, 240(column)×320(row) pixels in a display area 100. The numberof gate lines 31 is the sum of the number of gate lines 31 acorresponding to the reflective region 21, and the number of gate lines31 b corresponding to the transmissive region 22, amounting to 640 inthis example. The LCD driver 101 includes a line memory 111 having amemory capacity of a single row or more, and a gray-scale-levelconverter (γ-converter) 112 disposed for writing data in thetransmissive region 21. The LCD driver receives 101 an external timingsignal TG, and serial data signals Rn, Gn, Bn each including a digital8-bit ROB signal for each pixel

The LCD driver 101 in the present embodiment includes agate-timing-signal generator and a data-timing-signal generator (bothnot shown in the figure) for generating respective timing signals basedon the external timing signal. For generating the timing signals in theLCD driver 101, the timing signals for a single row of the pixels areseparated into two timing signal series including a timing signal seriesfor the reflective region 21 and a timing signal series for thetransmissive region 22. These timing signals are used for driving thegate lines 31 a and gate lines 31 b. The gate signals supplied to thegate liens 31 a, 31 b are generated in the LCD driver 101, or may begenerated in a shift register disposed on the TFT substrate.

The gray-scale-level conversion circuit 112 includes a look-up table forgenerating a gray-scale level for the transmissive region 22 based onthe gray-scale level for the reflective region received from theexternal circuit. More specifically, the LCD driver 101 temporarilystores the received pixel data in the line memory 111. At the timingTg(R) for writing data in the reflective region 21, the LCD driver 101converts the received pixel data signals into parallel analog signals,by using a serial-to-parallel conversion and a digital-to-analog (D/A)conversion without using the gray-scale-level conversion circuit 112,ant outputs the analog pixel signals to the data lines 32 via amultiplexer (MUX) 113. At the timing Tg(T) for writing data in thetransmissive region 22, the LCD driver 101 allows the gray-scale-levelconverter 112 to convert the received pixel data stored in the linememory 111 into inverted pixel data, then performs a serial-to-parallelconversion and a D/A conversion, and outputs the analog pixel signals tothe data line 32 via the multiplexer 113. The gray-scale-level converter111 may perform a γ-conversion in addition to the gray-scale levelconversion by using a look-up table in order to obtain similar γcharacteristics in the data for both the reflective region 21 andtransmissive region 22.

For example, if a pixel data signal K(n,m)=0 is received in the LCDdriver 101 for a K-th pixel disposed at an n-th row and am m-th column,the LCD driver performs a D/A conversion to the zero gray scale data(R(n,m)=0) at the timing Tg(R) for writing data into the reflectiveregion 21 of the K-th pixel, and outputs the corresponding analog data,such as a zero-volt or 10-volt signal, to the data line 32. On the otherhand, at the timing Tg(T) for writing the data into the transmissiveregion 22 of the same K-th pixel, the LCD driver 101 allows thegray-scale-level converter 112 to convert the pixel data signal K(n,m)=0into K(n,m)=255, performs serial-to-parallel conversion and D/Aconversion to the converted data K(n,m)=255, and outputs thecorresponding analog data, such as a 5-volt signal, to the data line 32.

FIG. 20 shows a driving-signal waveform for both the reflective region21 and transmissive region 22 at a specific stage of operation in theLCD device. The driving signal depicted therein includes a gate signalsupplied to the gate lines 31 a, 31 b and a data signal supplied to thedata line 32. In this example, a dot inversion driving scheme is usedand the common electrode signal is constant. The writing period for asingle pixel (or single line) is divided into a first writing period forwriting data into the reflective region 21, and a second writing periodfor writing data into the transmissive region 22, whereby the gate lines31 a and 31 b are driven by a high-level gate signal at differenttimings. The TFT 33 for the reflective region 21 is turned ON at thefirst timing Tg(R) or first writing period during which the gate line 31a is applied with a high-level potential, and writes the data suppliedthrough the data line 32 into the pixel electrode 35 in the reflectiveregion 21. The TFT 34 for the transmissive region 22 is turned ON at thesecond timing Tg(T) or second writing period during which the gate line31 b is applied with a high-level potential, and writes the datasupplied through the data line 32 into the pixel electrode 36 in thetransmissive region 22.

If a zero gray-scale-level data (dark-state data) is received for thepixel, a 10-volt data is supplied to the data line 32 at the timingTg(R) of writing the data into the reflective region 21, and the TFT 33corresponding to the reflective region 21 is turned ON, whereby the10-volt data signal is written into the pixel electrode 35. In thiscase, if the potential of the COM line 39 a is fixed at 5 volts, the LClayer 13 in the reflective region 21 is applied with an electric fieldcorresponding to the 5 volts, whereby the reflective to region 21operating in the normally-white mode assumes a dark state for the imagedisplay. On the other hand, at the timing Tg(T) for writing the datainto the transmissive region 22, the data line 32 is supplied with a5-volt data, and the TFT 34 corresponding to the transmissive region 22is turned ON, whereby the 5-volt data is written into the pixelelectrode 36. Since the common electrode 38 is applied with 5 volts, theLC layer 13 in the transmissive region 22 is not applied with anelectric field, whereby the transmissive region 22 operating in thenormally-black mode assumes a dark state for the image display.

In the present embodiment, as described above, the gate lines 31 in theLCD device include gate lines 31 a for the reflective region 21 and gatelines 31 b for the transmissive region 22, and the writing period forthe pixel includes two separate writing periods, whereby the common datalines 32 can supply different data signals to the reflective region 21and transmissive region 22. One of the regions 21, 22 receives a datasignal generated based on the received pixel data in the LCD driver 101,whereas the other of the regions 21, 22 receives a data signal generatedbased on an inverted data generated from the received pixel data by thegray-scale-level converter 112. This configuration provides differentpotential differences to the reflective region 21 and transmissiveregion 22 without increasing the number of data lines for writing datainto the pixel, the different potential differences allowing both theregions 21, 22 to represent similar gray-scale levels irrespective ofthe different normal modes.

FIG. 21 shows a schematic top plan view of a TFT substrate in atransflective LCD device according to a third embodiment of the presentinvention. The LCD device, generally designated by numeral 10 b, has asectional structure similar to that in the LCD device 10 of the firstembodiment, and includes first polarizing film, counter substrate, LClayer, TFT substrate, and second polarizing film. The polarizing axis ofthe first and second polarizing films and the longer axis of the LCmolecules in the present embodiment are also similar to those in the LCDdevice of the first embodiment. The LCD device of the present embodimentis different from the LCD device of the first embodiment in the planarstructure in the pixel, and the scheme for signal transfer via the gatelines and data lines.

As understood from FIG. 21, a plurality of gate lines 31 extending inthe row direction and a plurality of data lines 32 extending in thecolumn direction are formed on the TFT substrate. TFTs 33, 34 areprovided in the vicinity of each of the intersections between the gatelines 31 and data lines 32. The gate lines 31 for each row of the pixelsinclude a gate line 31 a connected to the gate of the TFTs 33, and agate line 31 b connected to the gate of the TFTs 34. The TFTs 33 eachhave a source/drain path connected between a data line 32 and the firstpixel electrode 35 provided in the reflective region 21, whereas theTFTs 34 each have a source/drain path connected between the same dataline 32 and the second pixel electrode 36 in the transmissive region 22.The common electrode 39 formed in common to the reflective region 21 andtransmissive region 22 is connected to a single common electrode (COM)line 40, which supplies a common electrode signal to all the pixels ofthe LCD device 10 a.

FIG. 22 shows the overall configuration of the LCD device 10 b of FIG.21 including a LCD driver 101 a. The LCD device 10 b of the presentembodiment is similar to the LCD device 10 a of the second embodimentexcept that the pixel electrodes 35, 36 are supplied with the same datasignal whereas the potential of the COM line 39 a is changed at the timeinstant of half the writing period to thereby provide different voltagesto the reflective region 21 and transmissive region 22 of the LC layer13. The LCD device 10 b of the present embodiment need not have the linememory and gray-scale-level converter used in the second embodiment.

FIG. 23 shows a driving-signal waveform for both the reflective region21 and transmissive region 22 at a specific stage of operation in theLCD device. The driving signal depicted therein includes a gate signalsupplied to the gate lines 31 a, 31 b and a data signal supplied to thedata line 32. In this example, a dot inversion driving scheme is used.The writing period for a single pixel (or single line) is divided into afirst period for writing data into the reflective region 21, and asecond period for writing data into the transmissive region 22. The TFT33 for the reflective region 21 is turned ON at the first timing Tg(R)during which the gate line 31 a is applied with a high-level potential,and writes the data supplied through the data line 32 into the pixelelectrode 35 in the reflective region 21. The TFT 34 for thetransmissive region 22 is turned ON at the second timing Tg(T) duringwhich the gate line 31 b is applied with a high-level potential, andwrites the same data signal into the pixel electrode 36 in thetransmissive region 22. The LCD driver 101 a supplies a common electrodesignal at the first timing Tg(R) during which data is written into thereflective region 21, and an inverted common electrode signal at thesecond timing Tg(T) during which data is written into the transmissiveregion 22. For example, the common electrode signal assumes 5 volts atthe first timing Tg(R) and assumes zero volt at the second timing Tg(T).

For display of a dark state, the data signal assumes zero volt in anegative frame at the timing Tg(R) of writing the data into thereflective region 21, and the TFT 33 corresponding to the reflectiveregion 21 is turned ON, whereby the zero-volt data signal is writteninto the pixel electrode 35. In this case, since the potential of thecommon electrode 39 is 5 volts, the LC layer 13 in the reflective region21 is applied with an electric field corresponding to the 5 volts,whereby the reflective region 21 operating in the normally-white modeassumes a dark state for the image display. On the other hand, at thetiming Tg(T) for writing the data into the transmissive region 22, thedata line 32 is also supplied with the zero-volt data, and the TFT 34corresponding to the transmissive region 22 is turned ON, whereby thezero-volt data is written into the pixel electrode 36. Since thepotential of the common electrode 38 is inverted at this timing toassume zero volt, the LC layer 13 in the transmissive region 22 is notapplied with an electric field, whereby the transmissive region 22operating in the normally-black mode assumes a dark state for the imagedisplay.

In the above exemplified case, the reflective region 21 is driven for anegative frame. If the reflective region 21 is driven for a positiveframe, the common electrode 39 assumes zero volt during the first timingTg(R) for writing data into the reflective region 21, and assumes 5volts during the second timing Tg(T) for writing data into thetransmissive region 22. For display of a dark state, the data signalassumes 5 volts in a positive frame at the timing Tg(R) of writing thedata into the reflective region 21. The pixel electrode 35 in thereflective region 21 is applied with the 5-volt data by turn of the TFT33 at the timing Tg(R), with the potential of the common electrode 37being zero volt, whereby the LC layer in the reflective region isapplied with an electric field corresponding to 5 volts to represent adark state. The pixel electrode 36 in the reflective region 22 is alsoapplied with the 5-volt data at the timing of Tg(T), with the potentialof the common electrode 37 being inverted to 5 volts, whereby the LClayer 13 in the transmissive region 22 is applied with no electric fieldto thereby represent a dark state.

Thus, both the reflective region 21 and transmissive region 22 representa dark state in the negative and positive frames.

In the present embodiment, as described above, the writing period forthe pixel is divided into the first timing and the second timing, boththe pixel electrodes 35 and 36 are supplied with the common voltage, andthe potential of the common electrode 39 is inverted between the firsttiming and the second timing. This configuration provides differentpotential differences to the reflective region 21 and transmissiveregion 22 without generating different data signals for the reflectiveregion 21 and transmissive region 22 the different potential differencesallowing both the regions 21, 22 to represent similar gray-scale levelsirrespective of the different normal modes.

FIG. 24 shows schematic section view of a transflective LCD deviceaccording to a second embodiment of the present invention. The LCDdevice 10 a of the present embodiment is similar to the LCD device ofthe first embodiment except that λ/2 wavelength films 18 and 19 areinterposed between the first polarizing film 11 and the countersubstrate 12 and between the TFT substrate 14 and the second polarizingfilm 15, respectively. The λ/2 wavelength films 18, 19 have respectiveoptical axes within the plane parallel to the substrates which areperpendicular to one another. The λ/2 wavelength films prevents theimage of a dark state from being observed to include blue color.

FIG. 25 shows a table showing the possible combination of the opticaltransmission axis of the first and second polarizing films 11, 15,longer axis of the LC molecules in the LC layer 13, and optical axis ofthe λ/2 wavelength films within the plane parallel to the substrates inthe LCD device. In this combination, the polarized direction of thelight passed by the second polarizing film 15 and the λ/2 wavelengthfilm 19 and incident onto the LC layer 13 is set parallel or normal tothe longer axis of the LC molecules in the LC layer 13. Thisconfiguration is employed so as to suppress the leakage light in thetransmissive region during display of a dark state.

A simulation was conducted to each combination tabulated in table 1shown in FIG. 25, thereby obtaining the results shown in FIG. 26. FIG.25 shows that the fifth and seventh combinations have lower leakagelight especially in the short wavelength region or blue color wavelengthregion.

The seventh combination is applied to the LCD device 10 c of the secondembodiment, which exhibits the polarized state shown in FIG. 21. Thefunction of this LCD device will be described hereinafter during displayof a dark state and display of a bright state.

Display of a Dark State

For display of a dark state in this embodiment, the driving signalsshown in FIGS. 3A and 3B are used so as to rotate the longer axis of theLC molecules in the LC layer 13 in the reflective region 21 by 45degrees, and maintains the longer axis of the LC molecules in thetransmissive region 22 at 90 degrees. In FIG. 21, dotted line representsthe direction of the polarized light, and the solid arrows represent theoptical absorption axis.

In the transmissive region 22, a 135-degree linearly-polarized lightpassed by the second polarizing film 15 having an optical transmissionaxis at 135 degrees (and thus an absorption axis at 45 degrees) isrotated by an angle equal to double the difference between the polarizedangle (135 degrees) of the same and the angle (157.5 degrees) of theoptical axis at the λ/2 wavelength film 19 during passing through theλ/2 wavelength film 19. The light passed by the λ/2 wavelength film 19turns into a zero-degree linearly-polarized light, which is incidentonto the LC layer 13. The zero-degree linearly-polarized light passesthrough the LC layer 13 as it is, pass through the λ/2 wavelength film18 to shift to a 135-degree linearly-polarized light, and is incidentonto the first polarizing film 11. The first polarizing film 11 havingan optical transmission axis at 45 degrees blocks the incident lighttransmitted from the backlight source, to thereby represent a darkstate.

In the reflective region 21, the linearly-polarized light passed by thefirst polarizing film 11 having an optical transmission axis at 45degrees passes through the λ/2 wavelength film 18 to shift to a90-degree linearly-polarized light, and is incident onto the LC layer13. The 90-degree linearly-polarized light passes through the LC layer13 to shift to a counterclockwise-circularly-polarized light, and isreflected by the reflection film 16 to shift to aclockwise-linearly-polarized light. The clockwise-circularly-polarizedlight again passes through the LC layer to shift to a zero-degreelinearly-polarized light and is incident onto the λ/2 wavelength film18. The zero-degree linearly-polarized light passes through the λ/2wavelength film 18 to shift to a 135-degree linearly-polarized light,and is incident onto the first polarizing film 11, which blocks theincident light to represent a dark state

Display of a Bright State

For display of a bright state in FIG. 27, the LCD device is applied withdriving signals shown in FIGS. 5A and 5B, to rotate the orientation ofthe longer axis of the LC layer 13 in the transmissive region 21 by 45degrees, and maintains the orientation of the longer axis of the LClayer in the reflective region 21 at 90 degrees. In the transmissiveregion 22, a 135-degree linearly-polarized light passed by the secondpolarizing film 15 having an optical transmission axis at 135 degreespasses through the λ/2 wavelength film 19 to shift to a zero-degree (or180-degree) linearly-polarized light, and is incident onto the LC layer13. The zero-degree linearly-polarized light passes through the LC layer13 to shift to a 135-degree linearly-polarized light, passes through theλ/2 wavelength film 18 to shift to a 45-degree linearly-polarized light,and is incident onto the first polarizing film 11, which passes theincident light to thereby represent a bright state.

In the reflective region 21, a 45-degree linearly-polarized light passedby the first polarizing film 11 passes the λ/2 wavelength film 18 toshift to a 90-degree (or 270-degree) linearly-polarized light, and isincident onto the LC layer 13. The 90-degree linearly-polarized lightpasses through the LC layer 13 as it is, and is reflected by thereflection film 16 to be again incident onto the LC layer 13. The90-degree linearly-polarized light passes the LC layer 13 as it is, andpasses through the λ/2 wavelength film 18 to shift to a 45-degreelinearly-polarized light. The first polarizing film 11 passes the45-degree linearly-polarized light, to represent a bright state.

The λ/2 wavelength films 18, 19 may be configured by a single-axialwavelength film, a biaxial wavelength film, or a combination of layeredsingle-axial wavelength film and a biaxial wavelength film. A simulationwas conducted to obtain the viewing angle dependency of the luminanceand the contrast ratio during display of a dark state, for the caseusing a single-axial wavelength film. FIGS. 28A and 28B show the resultsof simulation. For the case using the single-axial wavelength film, asshown in FIG. 28A, leakage light is noticed as observed from asignificant viewing angle in the orientation aligned with the directionof the λ/2 wavelength films 18, 19. This leakage light has an influenceon the contrast ratio being considerably reduced depending on theobserved direction, as shown in FIG. 28B.

Simulation was conducted for obtaining the viewing angle dependency ofthe luminance and contrast ratio during display of a dark state for thecase using a layered structure including a single-axial λ2 wavelengthfilm and a biaxial λ/4 wavelength film as the λ/2 wavelength films 18,19. In each of the λ/2 wavelength films 18, 19, the single-axialwavelength film is disposed near the polarizing film 11, 15 and thebiaxial wavelength film is disposed near the LC layer 13 in thesimulation. FIGS. 29A and 29B show the result of the simulation for theluminance and the contrast ratio, respectively. The layered structurehas the advantage of reduced leakage color as shown in FIG. 29A comparedto the case using the single-axial wavelength film shown in FIG. 28A.This improves the viewing angle dependency of the contrast ratio asshown in FIG. 29B.

Another simulation was also conducted for obtaining the viewing angledependency of the luminance and contrast ratio for the case using abiaxial wavelength film. The results are shown in FIGS. 30A and 30B,similarly to FIGS. 23A and 23B. The biaxial wavelength film providesreduced leakage color, as shown in FIG. 30A, compared to the case usingthe layered structure as shown in FIG. 29A. This also considerablyimproves the viewing angle dependency of the contrast ratio, as shown inFIG. 30B.

In the present embodiment, use of the λ/2 wavelength films 18, 19reduces bluish coloring during display of a dark state in the reflectiveregion, thereby improving image quality of the transflective LCD device.In addition, use of the layered structure including a single-axialwavelength film and biaxial wavelength film or a biaxial wavelength filmreduces the leakage light in the slanted viewing angle to therebyimproved the viewing angle dependency of the luminance and contrastratio. The other advantages are similar to those achieved in the firstembodiment.

In the first embodiment, a portion of the reflection film is notdisposed directly behind the pixel electrode 35 and first commonelectrode 37. However, the present invention is not limited to thisexample. The reflection film may be such that shown in FIG. 31, whereinthe reflection film 16 has a flat surface directly behind the pixelelectrode 35 or first common electrode 37.

In the above embodiments, IPS-mode LCD device is exemplified as the LCDdevice of the embodiments. The display mode of the LCD device of thefirst invention, for example, may be a fringe-field-switching(FFS)-modeinstead. FIG. 32 shows a sectional view of the FFS-mode LCD deviceaccording to a fourth embodiment of the present invention. The LCDdevice, generally designated by numeral 10 d, includes a reflectiveregion 21 and a transmissive region 22. On the TFT substrate 14 a, areflection film 16 and an embedding insulation film are formed in thereflective region 21. The reflection film 16 reflects the light incidentfrom the first polarizing film 11. The reflection film 16 has an unevensurface in general for improving the light dispersion effect; however, adispersion film may be additionally provided in the counter substrate 12instead of providing the uneven surface to the reflection film 16. In afurther alternative, a dispersion adhesive layer wherein lightdispersion beads are dispersed may be provided on the surface of thepolarizing film 11 near the counter substrate 12.

FIG. 33 shows a sectional view of the IPS-mode LCD device 10 of thefirst embodiment. Comparing the structure of FIG. 32 against thestructure of FIG. 33, the FFS-mode LCD device 10 d does not include acommon electrode 37 juxtaposed with the pixel electrode m35, differentlyfrom the IPS-mode LCD device 10. The FFS-mode LCD device 10 d includes areflection film 16 connected to a first common electrode line (notshown) and thus acting as the common electrode 37 in the reflectiveregion 21. In the transmissive region 22 of the FFS-mode LCD device 10d, a transparent common electrode 20 corresponding to the commonelectrode 38 in the IPS-mode LCD device 10 is provided at the rear sideof the pixel electrode 36 in the transmissive region 22. In the FFS-modeLCD device 10 d, the pixel electrode 36, underlying common electrode 20and reflection film 16 generates an electric field therebetween to drivethe LC layer 13. The driving operation of the FFS-mode LCD device 10 dis similar to that of the IPS-mode LCD device 10 of the presentembodiment, and thus omitted here for description thereof.

In the LCD device of the fourth embodiment, the structure similar tothat used in the first embodiment is used. The configuration of thethird embodiment may be combined with the configuration of the secondembodiment. Further, the FFS-mode LCD device may have a structuresimilar to the structure of the first through fourth embodiments.

In accordance with the transflective LCD device of the embodiment of thefirst aspect of the present invention, the reflective region and thetransmissive region of the LC layers are applied with different electricfields so that both the regions represent similar gray-scale levelsirrespective of operating in the different normal modes, thereby solvingthe image-inversion problem encountered in the conventionaltransflective LCD device.

It is preferable that the first common signal and the second commonsignal be inverted in synchrony with the pixel signal, and the firstcommon signal be substantially an inverted signal of the second commonsignal. For example, if the pixel electrode in both the reflective andtransmissive regions is applied with 5 volts, the first common electrodeis applied with a first common signal of zero volt, and the secondcommon electrode is applied with 5 volts. This allows the LC moleculesonly in the reflective region are rotated, whereby the image-inversionproblem can be solved. It is to be noted that the first common signalneed not be a strict inverted signal of the second common signal. Forexample, if the first common signal assumes zero volt or 5 volts, thesecond common signal may assume 6 volts or zero volt.

It is also preferable that the pixel electrode include a first pixelelectrode in the reflective region and a second pixel electrode in thetransmissive region, and each of the pixels be associated with a firstswitching device for coupling a data line to the first pixel electrode,and a second switching device for coupling the data line to the secondpixel electrode. A concurrent turn-ON of the first and second switchingdevices allows the common pixel signal to be supplied to both thereflective region and the transmissive region. After the supply of thecommon data signal, the first and second switching devices are turnedOFF, to allow the first and second pixel electrode to assume differentpotentials.

It is also preferable that the reflective region includes therein areflection film having a potential substantially equal to a medianbetween a potential of the first pixel electrode and a potential of thefirst common electrode. This suppresses an excessive electric field frombeing applied between the reflective film and the pixel electrode or thefirst common electrode, to reduce leakage light during display of a darkstate.

The potential of the reflection film may be determined by a capacitivecoupling between the same and the first pixel electrode and a capacitivecoupling between the same and the first common electrode. In analternative, the potential of the reflection film may be determined by apotential setting circuit.

It is also preferable that a portion of the reflection film is omittedin an area directly behind the first pixel electrode and the firstcommon electrode. In an IPS-mode LCD device, the reflective film maygenerate leakage light; however, this configuration reduces theluminance directly behind the electrode and thus reduces the leakagelight.

In an alternative, a portion of the reflection film in an area directlybehind the first pixel electrode and the first common electrode may havea flat surface, and the other portion of the reflection film may have anuneven surface. By suppressing the light dispersion, the luminance ofthe area directly behind the electrode can be reduced, whereby theleakage light is reduced.

In accordance with the transflective LC device of the embodiment of thesecond aspect of the present invention, the first and second switchingdevices write data into the first pixel electrode in the reflectiveregion and the second pixel electrode in the transmissive region,respectively. The first and second switching devices may write the samedata concurrently or separately in a time-division scheme into both theregions, while the common electrode in the respective regions hasdifferent potential. This allows the LC layer in the different regionsto be applied with different electric fields so that the image-inversionproblem can be solved.

In the second aspect of the present invention, at least one of thereflective region and the transmissive region may be driven by a lateralelectric field.

The at least one of the reflective region and the transmissive regionmay be driven in an in-plane-switching mode.

It is preferable in the second aspect of the present invention that eachof the pixels include a first common electrode in the reflective regionand a second common electrode in the transmissive region, and thereflective region include therein a reflection film having a potentialsubstantially equal to a median between a potential of the first pixelelectrode and a potential of the common electrode.

The potential of the reflection film may be determined by a capacitivecoupling between the same and the first pixel electrode and a capacitivecoupling between the same and the first common electrode. The potentialof the reflection film maybe determined by a potential setting circuitinstead.

It is preferable that a portion of the reflection film be omitted in anarea directly behind the first pixel electrode and the first commonelectrode.

In an alternative, a portion of the reflection film in an area directlybehind the first pixel electrode and the first common electrode may havea flat surface, and the other portion of the reflection film may have anuneven surface.

In the transflective LCD device of the second aspect of the presentinvention, at least one of the reflective region and the transmissiveregion of the LC layer may be driven in a FFS mode as well as in an IPSmode. In the FFS-mode LCD device, each of the pixels may include a firstcommon electrode in the reflective region and a second common electrodein the transmissive region, and the reflective region may include areflection film applied with a potential equal to a potential of thesecond common electrode.

In the transflective LCD device of the second aspect of the presentinvention, the reflective region and the transmissive region may bedriven in a normally-white mode and a normally-black mode, respectively.In this case, the LC layer should be applied with different electricfields in the reflective region and transmissive region by, for example,applying no electric field in the reflective region and a specificelectric field in the transmissive region for display of a dark state inboth the regions.

In the LCD device of the second aspect of the present invention, each ofthe pixels may include a first common electrode receiving a first commonelectrode signal common among the reflective regions of a plurality ofthe pixels, and a second common electrode receiving a second commonsignal which is common among the transmissive regions of the pluralityof the pixels. In this case, the first pixel electrode and second pixelelectrode may receive the same data signal for display of similargray-scale levels.

The first common signal may be is substantially an inverted signal ofthe second common signal. For example, if the first and second commonsignals each are to assume a suitable voltage between zero volt and 5volts, the second common signal may assume 5 volts when the first commonsignal assumes zero volt.

The first and second switching devices may be turned ON in atime-division scheme, and the first pixel electrode may receive a firstpixel signal for driving the reflective region of the LC layer in anormally-white mode, and the second pixel electrode may receive a secondpixel signal for driving the transmissive region of the LC layer in anormally-black mode. In this case, the data lines may be common to thefirst pixel electrode and second pixel electrode to apply differentvoltages thereto.

At least one of the first pixel signal and the second pixel signal maybe created by a data converter including a line memory and a gray-scalelevel converter including a look-up table tabulating gray-scale leveldata. The external data is stored in the line memory and can be used asit is for the reflective region, for example, and can be used for thetransmissive region after conversion using the look-up table. Thelook-up table may be replaced by a gray-scale level converter configuredby a logic circuit.

In the above case, the first and second switching devices may be turnedON in a time-division scheme, the first pixel electrode and the secondpixel electrode may receive a common pixel signal, and each of thepixels may include a common electrode for receiving different commonelectrode signals during a first timing when the first electrode signalreceives the common pixel signal and a second timing when the secondelectrode receives the common pixel signal. The present invention can beapplied to the IPS-mode LCD device, FFS-mode LCD device and VA-mode LCDdevice.

Since the above embodiments are described only for examples, the presentinvention is not limited to the above embodiments and variousmodifications or alterations can be easily made therefrom by thoseskilled in the art without departing from the scope of the presentinvention.

1-3. (canceled)
 4. A transflective liquid crystal display (LCD) devicecomprising first and second polarizing films having polarizing axesperpendicular to one another, a liquid crystal (LC) layer interposedbetween said first polarizing film and said second polarizing film, saidLC layer defining an array of pixels each including a reflective regionand a transmissive region juxtaposed, wherein: LC molecules of said LClayer have a longer axis extending parallel to or normal to thepolarization direction of the light incident onto said LC layer in saidreflective region; and each of said pixels includes a pixel electrodereceiving a pixel signal which is common between said reflective regionand said transmissive region, a first common electrode receiving a firstcommon signal which is common among said reflective regions of aplurality of said pixels, and a second common electrode receiving asecond common signal which is common among said transmissive regions ofsaid plurality of said pixels, wherein said pixel electrode includes afirst pixel electrode in said reflective region and a second pixelelectrode in said transmissive region, and each of said pixels isassociated with a first switching device for coupling a data line tosaid first pixel electrode, and a second switching device for couplingsaid data line to said second pixel electrode, and wherein saidreflective region includes therein a reflection film having a potentialsubstantially equal to a median between a potential of said first pixelelectrode and a potential of said first common electrode.
 5. The LCDdevice according to claim 4, wherein said potential of said reflectionfilm is determined by a capacitive coupling between the same and saidfirst pixel electrode and a capacitive coupling between the same andsaid first common electrode.
 6. The LCD device according to claim 4,wherein said potential of said reflection film is determined by apotential setting circuit.
 7. The LCD device according to claim 4,wherein a portion of said reflection film is omitted in an area directlybehind said first pixel electrode and said first common electrode. 8.The LCD device according to claim 4, wherein a portion of saidreflection film in an area directly behind said first pixel electrodeand said first common electrode has a flat surface, and the otherportion of said reflection film has an uneven surface. 9-10. (canceled)11. A transflective liquid crystal display (LCD) device comprising aliquid crystal (LC) layer defining an array of pixels arranged in amatrix, each of said pixels including therein a reflective region and atransmissive region juxtaposed, wherein: each of said pixels includes afirst pixel electrode in said reflective region, and a second pixelelectrode in said transmissive region; and each of said pixels isassociated with a first switching device for coupling together saidfirst electrode to and a data line supplying a data signal, and a secondswitching device for coupling together said second electrode and saiddata line, further comprising a first common electrode including aplurality of common electrodes connected in common and disposed in saidreflective regions of a plurality of said pixels, and a second commonelectrode including a plurality of common electrodes connected in commonand disposed in said transmissive regions of a plurality of said pixels,wherein said first and second switching devices are connected to acommon data line.
 12. (canceled)
 13. A transflective liquid crystaldisplay (LCD) device comprising a liquid crystal (LC) layer defining anarray of pixels arranged in a matrix each of said pixels includingtherein a reflective region and a transmissive region juxtaposed,wherein: each of said pixels includes a first pixel electrode in saidreflective region, and a second pixel electrode in said transmissiveregion; and each of said pixels is associated with a first switchingdevice for coupling together said first electrode to and a data linesupplying a data signal, and a second switching device for couplingtogether said second electrode and said data line, further comprising afirst gate line for controlling said first switching devices and asecond gate line for controlling said second switching devices, whereinsaid first and second switching devices are connected to said data linein common.
 14. A transflective liquid crystal display (LCD) devicecomprising a liquid crystal (LC) layer defining an array of pixelsarranged in a matrix, each of said pixels including therein a reflectiveregion and a transmissive region juxtaposed, wherein: each of saidpixels includes a first pixel electrode in said reflective region, and asecond pixel electrode in said transmissive region; and each of saidpixels is associated with a first switching device for coupling togethersaid first electrode to and a data line supplying a data signal, and asecond switching device for coupling together said second electrode andsaid data line, wherein said data line for each column includes a firstdata line connected to said first switching devices, and a second dataline connected to said second switching devices.
 15. The LCD deviceaccording to claim 13, further comprising a data converter forconverting a first gray-scale level of a data signal received fromoutside of said LCD device into a second gray-scale level to be suppliedto one of said first pixel electrode and said second pixel electrode.16. The LCD device according to claim 15, wherein said data converterincludes a memory for storing therein said data signal received fromoutside, and a gray-scale level converter for converting said firstgray-scale level of said data signal into said second gray-scale level,and writes said second and first gray-scale levels into said one and theother, respectively, of said first pixel electrode and said second pixelelectrode.
 17. The LCD device according to claim 16, wherein saidgray-scale level converter converts said first gray-scale level intosaid second gray-scale level by using a look-up table.
 18. The LCDdevice according to claim 17, wherein said look-up table tabulates saidfirst gray-scale level and corresponding second gray-scale level inassociation so that said first gray scale level assumes a maximum whensaid corresponding second gray-scale level assumes a minimum.
 19. TheLCD device according to claim 13, wherein said gray-scale levelconverter is configured by a logic circuit.
 20. The LCD device accordingto claim 13, further comprising a single common electrode including aplurality of common electrodes disposed in said reflective regions andsaid transmissive regions of a plurality of said pixels.
 21. The LCDdevice according to claim 20, wherein a potential to be written in saidsingle common electrode is inverted at a timing of switching for writingdata through said first switching device or said second switchingdevice.
 22. A method for driving a transflective liquid crystal displaydevice (LCD) including a reflective region and a transmissive region ineach of pixels arranged in an array, said method comprising the stepsof: generating a first data signal and a second data signal havingtherebetween a specific potential relationship; and applying said firstdata signal and said second data signal to said reflective region andsaid transmissive region, respectively.
 23. The method according toclaim 22, wherein said relationship between said first data signal andsaid second data signal is such that said first data signal assumes amaximum gray-scale-level potential when corresponding said second datasignal assumes a minimum gray-scale-level potential.
 24. The methodaccording to claim 22, further comprising the step of applying a firstcommon electrode signal to a first common electrode disposed in saidreflective regions of a plurality of said pixels and a second commonelectrode signal to a second common electrode disposed in saidtransmissive regions of a plurality of said pixels, said first commonelectrode signal having a potential different from a potential of saidsecond common electrode signal.
 25. The method according to claim 22,wherein said LCD device includes a first switching device for coupling adata line to said first pixel electrode and a second switching devicefor coupling said data line to said second pixel electrode, a first gateline for controlling said first switching device, a second gate line forcontrolling said second switching device, said method further comprisingthe steps of: turning ON said first and second switching devices in atime-division scheme, to apply a common data signal to said first andsecond pixel electrodes; applying a first common electrode signal to acommon electrode during applying said common data signal to said firstpixel electrode; and applying a second common electrode signal to saidcommon electrode during applying said common data signal to said secondpixel electrode, said first common electrode signal having a potentialdifferent from a potential of said second common electrode signal. 26.The method according to claim 22, wherein said LCD device includes afirst switching device for coupling a first data line to said firstpixel electrode, a second switching device for coupling a second dataline to said second pixel electrode, said method further comprising thestep of: applying said first and second data signals to said first andsecond data lines, respectively.
 27. The method according to claim 26,wherein one of said first and second data signal is supplied fromoutside of said LCD device, and the other of said first and second datasignals has a gray-scale level converted from a grays-scale level ofsaid one of said first and second data signals by using a look-up table.28. The method according to claim 27, wherein said look-up table is suchthat similar γ-characteristics are obtained for both said reflective andtransmissive regions.
 29. The method according to claim 22, wherein saidLCD device includes a single common electrode for both said reflectiveregion and said transmissive region of a plurality of said pixels, saidmethod further comprising the steps of: applying said single commonelectrode with a first common electrode signal at the timing of writingsaid first data signal; and applying said single common electrode signalwith a second common electrode signal at the timing of writing saidsecond data signal.